Leon Panetta, in Japan on Monday for his first visit as Defense secretary, slammed North Korea’s nuclear weapons program and criticized an increasingly assertive China that is rapidly modernizing its military with a "troubling" lack of transparency.

Panetta, in an op-ed that appeared in Japan’s largest-circulation newspaper, Yomiuri Shimbun, on Monday, said that Washington and Tokyo share common challenges in the Asia-Pacific region. “These include North Korea, which continues to engage in reckless and provocative behavior and is developing nuclear weapons and ballistic missiles, which pose a threat not just to Japan, but to the entire region,” he wrote.

Panetta said the U.S. would work with Japan to bring North Korea back to the six-party nuclear talks with China, Russia, and South Korea. In a sign that North Korea is chafing under its international isolation and crippled economy, Pyongyang sent a delegation to Geneva this week to continue talks with U.S. officials about possible steps toward denuclearization contained in past U.N. Resolutions. Though there has been relatively little movement on the issue since the 2005 six-party talks, recently North Korea has reopened diplomatic channels, reaching out to South Korean counterparts for talks and meeting with U.S. officials in New York in late July.

Washington will also work with Japan to encourage China to play a “responsible role” in the international community, Panetta wrote. Amid fears in Washington that Beijing is developing stealth warplanes and other sophisticated weapons as a way of one day challenging U.S. dominance in the Pacific, Panetta wrote that “China is rapidly modernizing its military, but with a troubling lack of transparency, coupled with increasingly assertive activity in the East and South China Seas.”